In case a circulating blood flow decreases by some cause in, for example, a patient in intensive care unit (ICU), the blood that has been supplied to the organs in the abdominal cavity is redistributed to the other important organs. As a result, the blood flow in the mucous membrane of the alimentary canal begins to decrease earlier than the other organs to fall into hypoperfusion. It follows that the mucous membrane falls into hypoxemia, which leads to acidosis of the tissue. The mucous tissue is then destroyed to allow colonic bacteria or endotoxin into the body fluids, causing sepsis or multiple organ failure. That is, early finding and early treatment of hypoperfusion in the alimentary canal could remove one of the causes of multiple organ failure, which is said to be a main cause of death in ICU. In order to detect an increase in carbon dioxide partial pressure due to hypoperfusion, a sensor for a carbon dioxide partial pressure in an alimentary canal has been proposed, which is to be fitted to the tip of a stomach tube or an ileus tube inserted into the alimentary canal for medical treatment, as disclosed in JP-A-7-231885 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). The carbon dioxide gas sensor to be used includes the one described in JP-A-61-144562, which detects the concentration of carbon dioxide dissolved in a bicarbonate buffer solution (e.g., NaHCO.sub.3 solution) as a change in hydrogen ion concentration by means of an Ion-Sensitive Field-Effect Transistor (ISFET).
However, the conventional biological gas sensor of this type tends to indicate a higher level than the actual carbon dioxide partial pressure, failing to make accurate measurement, because the hydrogen ion concentration of a bicarbonate buffer solution contained in the sensitive part is liable to increase in the presence of hydrogen sulfide gas or compounds thereof and/or weak acids or gas thereof which exist in the alimentary canal.